It is already known in the art to equip agricultural harvesters with apparatus for establishing the mass flow rate of crop material in order to gather data on the amount of harvested material over a time period and on the local yield rate in distinct areas of a field. Such apparatus has already been described with respect to combine harvesters in U.S. Pat. No. 5,343,761 and European Pat. No. EP-A-0 501 099. In U.S. Pat. No. 5,343,761 the mass flow rate is derived from the measurement of the forces resulting from the impact of grain kernels on a vertical plate which is installed in a clean grain elevator. The left and right edges of the impact plate are spaced from the elevator walls to prevent lodging of material other than grain between these edges and the walls. Consequently small particles may reach the area behind the impact plate and accumulate on the rear of the plate, thereby affecting the zero load of the apparatus and distorting the mass flow readings. Such apparatus requires cleaning at regular intervals.
The apparatus disclosed in European EP-A-0 501 099 uses variations in capacitance caused by a flow of grain to establish the momentary flow rate. This device comprises no movable sensor portions whereof the displacement under action of the mass flow is measured.
Another type of measuring device is shown in European Pat. No. EP-A-0 753 720. Herein the flow of crop material is guided along a curved surface which is mounted for pivotment about a transverse axis. A gap is provided between the curved surface and the housing of the grain elevator to enable free oscillation of the surface in its normal measurement range. The inlet section of the surface has to be dimensioned slightly larger than the dimension of the outlet of the grain elevator to preclude false readings, caused by impact on the edge, and loss of conveyed crop material. In the forage harvester illustrated in the same publication a sensor member for monitoring the mass flow rate has been installed in an aperture in the spout. The inlet portion of the sensor member is installed over the front edge of the aperture and its outlet portion extends over the rear edge of the same aperture in order to maintain all material within the spout. The gap between the body of the spout and the sensor member is a critical area for pollution by stray material, as a plugging thereof will result in false readings of the measuring apparatus.